Peak leveling circuit



Oct. 23, 1951 R. c. MOORE PEAK LEVELING CIRCUIT Filed May 24, 1949 INVENTOR.

" maf/W c. Moo/@f Patented Oct. 23, 19h51 PEAKLEVEIIING CIRCUIT" rationfpflennsylvania Application May-24; 1949, Serial No. 95,052

1 ,Claima .(1 (Cl. 178.-'13) ThemresentAinventiomrelates.ttelevisionzrefe. t ceiving systems, and more particularly relates stoz. an improvedircuit-gior. reinserting in .suche sysef, tems thedirectecurrentacomponent .of ithe compositeyideo. signal. 5

In .present- .day..television ytransmitting .systems s.; a. cameraA or .pick-up .tubeis frequently utilized to;- k: convert ether optical image .to ,be televised: into..l a corresponding;V 1 image in stored velectrostatic charge;l This;.charge;image,-.which.is lformed ony 10 theY mosaic--of the camera; tube,Y is Vthen scanned f. v

by an e1ectron-..beamto develop an output voltage variation .the instantaneous :1 value.- ot which.- is.A dependent uponfthe electrostatic.: potential ,pos-L sessed Icythe particular. areaiofvthe mosaick then l5 underc-thef..scanningabeams This potential, 1in.' turn,y Vis representative.4 ofthe -lorillianceof-I a spe-L V cicrpointv-.on thel opticalziimage-.`

In .order a. that the output voltageefromy the# camera.v tube.;may.contain @all @tithe-information e 20 derivedfrom suchiscanning-of fthe=mosaic,.it mustrI 1 vary overfaLwidesband offrequenciesw This-band mayhave an. upper limit thekorder oi 'sifirmega-Y s cycles, forcexampla'andalower Alimit asfar down -v as directecurrentthat-iszero frequency- .While 25 the.y upper portionof this-frequencybandconveys the velectrical:.'counterpart 1of=fvariations \inlight# intensity v. occurring from oneeelemental areawof f the .opticallimage to the-next, -the-lower-frequenicies; onthe.- other handy are thosefwhichvcontain 30 informationfregardingthe backgroundfillumina-f tion of the image. In other words, they-indicate its average brillianceasfopposed -to-its2detailedfillumination; Accordingly; an-*image-that-is pre--- dominantly. blackA or darkin colora-will produce a- 35 low-frequency component which Vis diierent I fromthat which-is produced by :an image-havingc a predominantly light background.

It is, of ecourse; necessaryffthat the -highv-fre-J quenciesin the video signallfloe adequatelyltransi .10 mittedfA and received: inA order to .reproduceL the l pictureldetails. l It is s equallynecessary' hovveveiaA thatthe lowfrequencies the Avideo -signalgwhichh represent the averagebril-liance ofthepicturefbe; Similarly r transmittedand ireceived so that theA 45 reproduced; image. vvill` abe a` true. representati'onf-v-A y of the original subject.

Under presentfstandards of transmissions each@ cycle ofI the :composite televisionsignalincludesia: if portion whichais utilized-1 itcI conveyethe @video in'-\ 50 formation, and another :portionfinl whichilan imv1 L 'y pulse is transmittedvvhiclr isrutilizedfjto synchro-f l hZe :the VOperation.:oifthe scanningfgeneratorsf atvV 1 the Vreceiver.Witlziche corresponding 'scanninggen-x craters;at.theA transmitterV Eachi suchi synchro-1I s 55 2 nizingjmpulse is-of rather short duration as com.-.-. pared, .tothe-video signal interval, and during this v ,Y synchronizing` period ,thertransmitter is operated at .its :maximum powerfoutput. In otherv Words, each;synchronizingfimpulse is transmitted at full-T carrier amplitude.:l 'The level of the blanking sig:- nals,..Which--.are transmittedduring the retrace periodioffeach--line-deection cycle, is arbitrarily senat-approximately 75 of this maximum transmitterrpower; whilesignals .representative of.. whitefinathe; image i are vtransmitted at values representative'V of lgreater degrees of? modulation.

It willthus be seen that both :the synchronizing impulses-,and the blanking signalsare of prede-1. terminedamplitude, and undergo no appreciable variation as longzasstheemaximum power output z of Ythe transmitter, .remainsasubstantially un,. Y

changed. Y Y

Accordingly, either, the .tips of fthe synchronizing pulses, or the blankingsignal pedestals upon..- Which' the. fsynchronizing pulses. are positioned (Whichllpe'destals .cincidewiththe black level of the signal) are suitablefor use as areference level f to whichtheralternating, current axis of the videou signal may be:related.f Apicture thatis-predominantly black, for example,.should.normally have its AJC. axis occupy a. position relatively closer` to thelevel. representedlby black than is the case* f with' al picturelthatuis `predominantly white.

The composite video signal output of the television transmitter has ythe A.C. axis of the lvideosignal properly relateclvto black level inthe vvave-4V form. rThis is due tofthe fact that the directcurrent component yof thev signal is normally r` added. atthe transmitter just prior to the pointat which the intelligence modulates'the conventional radio-frequencyaoscillator. Asia result, the low-frequencyv information, Whicli was removedY n from the output of the camera tube due to passage v of the'signal wave through the usual capacitively` ceupledyideo amplier circuits, is restored, Thus this D.C. component is present in the -transmittedsignal .and also at-the pointwhere such signal is picked up by a receiver.

The latter, however, lcustomarilycontains another f seriesv ofcapacitively-coupled video amplifierfstagesgewhich act to-rremove this D'.C. component; from the compositesignal'inthe same way that it Waslremovedby these similar units at the; transmitter. Consequently, somemeans must be provided-for restoring the low-frequency information to thesignals. lFailure to do so Would-mean Y that "the A.-C2.'v axis of the videoV signal is Vnot correctly 'positioned' with:z respect to a given ref erence voltage lsuche asfblack,* and, as a result,

each image reproduced by the receiver will contain the same average amount of light regardless of the overall brilliance of the particular object being televised at the transmitter. For example, when the direct-current component of the signal is not present, a scene having a large number of white areas appears much less brilliant on the screen of the image-reproducing tube, and similarly, a scene in which dark areas predominate appears to be considerably lighter in color than is actually the case. In other words, the insertion of a brightness component shifts the A.C. axis of the video signal with respect to a fixed reference level, thus varying the bias between the cathode and control electrode of the image-reproducing tube so as to alter the average brilliance of the reproduced image.

While it is customary to employ some form of so-called D.C. inserter circuit in commercial television receivers, nevertheless the operation of many of these circuits is rendered unsatisfactory by the reception of noise or other spurious impulses which may be received along with the video signal. Since a majority of these standard circuits employ a rectifying device in combination with a condenser to develop a charge on the latter the value of which is representative of the average value of the composite video signal, the presence of noise impulses of large amplitude may cause a relatively large variation in this developed voltage. Although most noise or other interference is of relatively short duration, nevertheless the fact that it may be of high amplitude causes the energy content thereof to be considerable. Thus in the presence of noise or other interference the conventional D.C. inserting circuit is caused to develop a voltage which is not truly representative of the D.C. information in the signal, resulting in an inaccurately reproduced image.

One object of the present invention, therefore, is to provide an improved form of D.C. inserting circuit for television receivers.

Another object of the invention is to provide a D.C. inserting circuit in which the position of the alternating current axis of the video signal is properly related to a given reference level, regardless of the presence of noise or other spurious impulses received during the video signal interval.

A further object of the invention residesin the provision of a D.C. inserting circuit for television receivers in which the amplitude of any noise occurring during the picture interval is limited substantially to the amplitude of the synchronizing impulses.

Other objects and advantages will be apparent from the following description of preferred forms of the invention and from the drawing, in which:

Fig. 1 is a schematic representation of a D.C. reinserting circuit in accordance with the present invention;

Fig. 2 is a graph illustrating the operation of the circuit of Fig. l; and

Fig. 3 is a modification of the circuit of Fig. l.

To the foregoing general ends it is a feature of the present invention to provide, in a television receiver wherein a composite television signal is present which lacks a D.C. component but which may include spurious impulses having a A peak amplitude greater than the peak amplitude of the televtsion signal itself, a clipping device for limiting the amplitude of the spurious impulses substantially to the peak amplitude of the television signal, a D.C. restorer circuit energized by the output of the clipping device, and means for applying the output of the D.C. restorer circuit to the clipping device to regulate its clipping level, whereby the spurious impulses are so limited in amplitude by the clipping device as not to affect to an appreciable extent the output of the D.C. restorer circuit and hence the clipping level of the clipping device.

Referring now to the drawing and to Fig. 1 in particular, there is shown an electron discharge tube V forming part of the video signal circuit of a television receiver. While tube V isshown at a tetrode having an anode, a cathode, a control electrode, and a screen electrode, it will be obvious as-the description proceeds that electron discharge devices having other numbers of electrodes `may readily be employed in place of the particular tube illustrated. 'I'he cathode of tube V is-grounded directly as shown, while the anode of the tube is connected to a source of positive operating Vpotential B+ through theload resistor R.

A composite television signal, including synchronizing pulses P and pictureinformation S, is impressed upon the control electrode of tube V through the coupling condenser C. This signal acts to modify the `electron flowv within tube V thereby to develop a varying output. voltage across the load resistor R. The output vvoltage v thus developed will be of the same general waveform as the input signal after circuit equilibriumY is reached, but will be reversed in polarity and will have an amplitude determined by the amplification factor of the tube, the plate resistance ofAV the tube, and the value of the load.l resistor R.

For inserting the proper direct-current com--l ponent in the output signal, a network is employed which includes a diode D, a condenser Ci and two resistors Ri and R2. A portion of the signal output of the tube V, as developed across resistor R, is appliedtothe anode of diodeD.

When the video signal output of'tube Vis applied to the anode of diode D, however, the latter is rendered conductive, and current ows in the diode circuit including resistors R1 and R2. This flow of diode current causes a potential drop across resistor R1 having the polarity indicated in the drawing. A condenser C1 lis connected in parallel with resistor R1, so that the potential across resistor R1 appears as av charge on condenser Ci. The time constant of, theelements C1R1 is preferably such that the charge on condenser Ci is maintained substantially constant during the rapid variations in'picture content that occur within a single line-scanning interval. This charge, therefore, is representative of the average brilliance of the optical image, inasmuch as the amount of energy rectified by the diode D is proportional to variations in the height of the synchronizing pulses P relative to the A.C. axis of the input signal. For example, a video signal containing a considerable amount of highlight illumination will have its AA..C. axis at a relatively greater'distance, from either black level or from 'tips than'wiu a picture the synchronizing"l 'pulse Y whichfis predominantiyedarkf The :volta-gef n condenser C1 therefore, varies directly with 'Varel'- iation in thej distance betweelrlA th'Aq-C.. axis of the input-signal to tube V and the tips ofthe synchronizing pulses contained', therein.

`The fixed control grid bias ofjtube'V in Figikluis supplied by the negative' bias source Ee through the, two series-connected"resistors R3 andR1thei former being of relatively large value and serving as the usual grid -rsi's't'r fdrthetube. Consequently,iwhen no signal is being passed by :the Y incoming signers. when such La signal ise'ceved by tube V, however, the bias on the control electrode of the tube is changed in a manner now to be set forth.

The portion of the video signal output of tube V which is applied to the anode of diode D is rectied, and a control voltage developed on condenser C1 which is substantially equal to the voltage differential between the tips of the synchronizing pulses P in the input signal and the A.C. axis thereof. This result is made possible by a suitable choice of values for resistors R, R1, and Rz, the latter being quite small relative to the other diode resistor R1. The potential drop across resistor R1, therefore, approximates the actual peak value of the applied signal.

However, the charge thus developed on condenser C1 is of such polarity as to oppose the negative bias voltage supplied to the control electrode of tube V from the bias source Ea The actual operating bias of tube V, therefore, is no longer that supplied by the source Ec alone, but is instead a voltage representing the difference between the output of source -Ec and the voltage developed across condenser C1.

'Ihe above mode of operation is perhaps more readily seen by reference to the graph of Fig. 2, in which the input signal wave is shown in relation to the operating characteristic (Ew-Ib) of tube V. As shown in this graph, the grid bias -Ec acts (in the absence of an incoming signal) to maintain tube V substantially at cut-off. Hence, it will be appreciated that application of the composite television signal to tube V (with the sync they would occupy diierent voltage level in the output signal as a result of variations in picture content.

However, by applying the varying voltage across resistor R1 to modify the xed bias -Ec, this undesirable mode of operation is overcome in the manner shown in Fig. 2. The actual operating bias of tube V in the circuit of Fig. 1 accordingly becomes the difference between the xed bias -Ec and the varying bias appearing across resistor R1. Since this differential bias varies with variations in the height of the sync pulses P, it acts in the manner shown by Fig. 2 to maintain the tips of these synchronizing pulses substantially at the cut-01T point of the amplifier tube. This produces a signal output in which the synchronizing pulses always occupy the same level, and in which the D.C. component of the transmitted signal has been reinserted so as to insure accurate image reproduction.

ralliement@ often results* inL the "development 'of' a kconsiderablyjAjgreated D. C. 'vol'tagethanis actually rep- ,Y'note'd, howevenvthat Yin applicants artends-beyond ythe cuto'iflevel of the tube. Furthermore, noise impulses such as N, while frequently of large amplitude, are likewise normally of short duration, and hence their energy content after clipping is rather low. Thus, the voltage on condenser C1, which is representative of the signal energy content, is not appreciably affected by the presence of these spurious shortduration impulses.

In Fig. 3 a modification of the circuit of Fig. 1 is illustrated wherein tube V is connected to act as a cathode follower, with the output voltage of the tube, as developed across thev cathode resistor R4, applied to the cathode of diode D through the condenser C1. In the circuit of Fig. 3, the anode resistor R of tube V is so chosen as to supbias across resistor R1.

on the control electrode of tube V. With resis tor R2 being made considerably smaller than resistor R1, most of the diode voltage will appearv across the latter, and will be effective in reducing the negative bias voltage of Ec as applied to the: control electrode of tube V through resistor R3: in the same manner as set forth in connection with the description of Fig. 1. The output of tube; V will be of negative polarity-that is, the synchronizing pulses will extend in a negative direction rather than in a positive direction as is the case in the circuit of Fig. 1.

The following values are illustrative of those which may be employed for the components indicated in Fig. 3. However, it will be appreciated that other values may be given to any or all of these elements as may seem desirable or necessary:

V, type 604 C, .05 mf.

R, 3300 ohms C1, .015 mf. R1, 2.7 megohms C2, .1 Inf.

Rz, 33,000 ohms -Ea -14 volts R3, 1 megohm R4 4700 ohms tive lofthe picture information itself. It

wevesisters connected between said source of ngative bias voltage and the control electrode of said amplier tube, a. third resistor, a diode having its cathode connected to a point intermediate said pair of resistors and its anode connected to said source of negative bias voltage through said third resistor, an impedance connected in the anode circuit of said amplifier tube and across which a voltage is developed during conduction of said tube, a condenser, means for applying the voltage developed across at least a portion of said impedance to the anode of said diode through said condenser, an output resistor connected between the cathode of said amplifier tube and ground, means for deriving from said output resistor a television signal output which contains a direct current component, and means for applying at least a portion of the television signal output of said amplifier tube as derived from said output resistor to the cathode of said diode. 20

ROBERT C. MOORE.

. REFERENCES `CITED The following references are of record in the Number Name Date 2,227,026 Schlesinger Dec. 31, 1940 2,254,114 Wilson Aug. 26, 1941 2,295,059 Smyth Sept. 8, 1942 2,525,103 Sprecher Oct. 10, 1950 FOREIGN PATENTS Number Country Date 46,154 France Mar. 6, 1936 1 444,049 Great Britain Mar. 12, 1936 505,899 Great Britain May 15, 1939 1 (Duplicate of French Ad. 17E/16,154n2/y 

